1. Field of Application
The present invention relates to a rotation angle detection apparatus that detects a degree of rotation of a rotary shaft from a reference angular position. In particular, the invention relates to a rotation angle detection apparatus which incorporates a magnet and in which the degree of rotation is detected based on changes in detected levels of magnetic flux.
2. Description of Related Art
Types of rotation angle detection apparatus are known in the prior art, which detect changes in rotation angle of magnets by utilizing magnetic sensors, and thereby detect amounts of change in rotation angle of a rotary shaft. A rotary shaft whose angular position is to be detected by such an apparatus will be referred to as the “output rotary shaft”. A type of rotation angle detection apparatus that enables detection of amounts of rotation angle (from a reference angular position) that exceed 360° has been described for example in Japanese patent publication number 2002-213910 (referred to in the following as reference document 1). Such a type of rotation angle detection apparatus will be designated in the following as an “over-360° rotation angle detection apparatus” in the following.
With the over-360° rotation angle detection apparatus described in reference document 1, two shafts carrying respective magnets engage respectively independently with an output rotary shaft (i.e., whose angular position is to be measured), and the respective rotation angles of the two magnet shafts are detected by corresponding magnetic sensors. Two rotation angle detection signals are thereby produced by these magnetic sensor,
With the apparatus of reference document 1, a first one of the rotation angle detection signals is a signal that varies monotonically over an entire range of measurement of the angular positions of the output rotary shaft. The second rotation angle detection signal varies in a periodic manner (i.e. successively changing by increasing from a minimum value to a maximum value) as the rotation angle of the output rotary shaft is successively increased throughout the measurement range, with a plurality of variation periods of the second rotation angle detection signal occurring over the measurement range.
Calculations are performed on the respective rotation angle detection signals from the two magnetic sensors, to obtain values of amounts of rotation of the output rotary shaft that exceed 360°.
However such a type of rotation angle detection apparatus is known to exhibit a lowering of detection accuracy as a result of the effects of temperature variations and of long-term usage. For that reason it has been proposed in Japanese patent publication number 2005-55297 (referred to in the following as reference document 2) to utilize a temperature detector to apply temperature compensation to a rotation angle detection apparatus of the type described in reference document 1. In addition it has been proposed in Japanese patent publication number 2004-53444 (referred to in the following as reference document 3) to store information beforehand, for the purpose of applying compensation for the effects of long-term usage to a rotation angle detection apparatus of the type described in reference document 1.
However such types of prior art rotation angle detection apparatus require the addition of temperature sensor elements (in the case of methods which apply temperature compensation), thereby increasing the overall cost of such an apparatus. In the case of an apparatus in which compensation is applied for the effects of long-term variations, actual changes in component characteristics as a result of long-term usage may vary substantially from predicted changes, due to the effects of manufacturing variations in the component characteristics, etc.
Hence, such prior art types of apparatus have the disadvantage of unreliability with respect to long-terminal accuracy of measurement. In addition, they require the use of a plurality of magnets and a mechanism which drives the magnets respectively separately for rotation, and so have the disadvantage of a relatively complex configuration, resulting in further increases in manufacturing costs.